Ice buildup on airplane wings is one of the aviation industry's most baffling and serious problems. Ice accumulation can reduce the lift of an airfoil in flight, and ice shed during takeoff can damage engine fan blades. Eight major airline accidents, resulting in casualties, since 1982 have been attributed entirely or in part to ice buildup. Early detection of ice can alert the crew to take remedial action before a dangerous situation develops. Several techniques for ice buildup detection currently exist, but most use external sensors that are exposed to the elements where they are subject to damage. See, e.g., "Wing Ice Detector," Lawrence Livermore National Laboratory Technical Production Group Report, October 1995, for a discussion of optical sensing methods; "Deicing-Fluid and Ice-Thickness Monitor For Aircraft," NASA Tech Briefs, page 54, September 1997, for a discussion of vibrating sensing elements; "Wing-Mounted Sensor Warns of Ice Accumulation," by Mark A. Gottschalk, Design News, page 153, Sep. 7, 1992, for a discussion of ionic conduction cells; and "Keeping Ice Off Airplane Wings," by Greg Paula, Mechanical Engineering Magazine, May 1997, for a discussion of electromagnetic sensors.
The petroleum and other chemical industry employ long sections of pipe for transfer of chemicals and for processing. Solid material buildup on the interior of these pipes can eventually block chemical flow within these pipes causing dangerous plant operating conditions or changing process conditions in dynamic reactors. Presently, one determines such blockage from a drop in pressure or a drop in flow rate.
In "Ultrasonic Aircraft Ice Detector Using Flexural Waves," U.S. Pat. No. 4,461,178, which issued to Jacques R. Chamuel, on Jul. 24, 1984, a system for the detection of wing icing by monitoring variations in flexural waves transmitted through the outer plate material of an aircraft airfoil is described. The basis of this approach is that compressional waves are not sensitive to ice accumulation on a metal plate, whereas the flexural modes are sensitive. A pulse is introduced into a metal plate and detected by a receiver at a distance. The pulse generates both compressional wave and flexural waves. Because the compressional waves travel faster than the flexural waves, it is possible to distinguish between the two waves based on arrival times using elaborate gating circuitry and peak detection ratio circuits. The ratio of the amplitude of the compressional wave to that for the flexural wave is thereby determined. This ratio changes with ice accumulation on the plate. The detected signal must be filtered so that aircraft structural vibrations can be removed. Although Chamuel states that the originally generated wave may be in the form of a single pulse, a burst of a preselected frequency, or a continuous wave, there is no teaching as to how to measure the requisite ratio for continuous wave excitation. Moreover, pulsed signals are likely to interfere with aircraft electronics.
In "Ice Detector," U.S. Pat. No. 4,604,612, which issued to Roger D. Watkins et al. on Aug. 5, 1986, a method that uses horizontally polarized Lamb waves to detect the presence of ice on a metal plate is described. A complex arrangement of six strip transducers is employed to generate this particular type of Lamb waves. A tone-burst approach is utilized.
In "Apparatus And Method For Detection Of Icing Onset And Ice Thickness," U.S. Pat. No. 5,095,754, which issued to David K. Hsu et al. on Mar. 17, 1992, it is stated that compressional waves do not adequately distinguish between transmission into ice and transmission into water, and that compressional waves propagating into a water layer and reflected at the water/air interface are practically indistinguishable from the ice/air interface. A buffer block is therefore embedded in the metal in order to delay the echo signal such that the receiver amplifier is not saturated in these pulse-echo measurements.
In "Ultrasonic Method And Apparatus For Detecting And Identifying Contamination Such As Ice On The Surface Of A Structure," U.S. Pat. No. 5,507,183, which issued to Francois Larue and Jerome Bisson on Apr. 16, 1996, an invasive method for detecting deposits on plate structures is described which requires the insertion of a wedge through a hole in the plate. The top surface of the wedge is the actual detecting surface rather than the plate itself. Pulse-echo techniques are employed, and the detection region is localized on the wedge.
Accordingly, it is an object of the present invention to provide a method for detecting ice buildup on airfoils.
Yet another object of the present invention is to provide a method for detecting material buildup on the inside of pipes.
Still another object of the invention is to provide a method for measuring changes in material buildup on metal surfaces.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.